1. Field of the Invention
This invention relates to a rubber product with lips, and more particularly to a rubber product of the type which includes a tubular body having a cross-sectionally corrugated inner surface, and a plurality of lips which are formed integrally on and project radially outwardly from an outer surface of the body in respective planes perpendicular to a longitudinal axis of the body, and are spaced at intervals along the longitudinal axis.
2. Description of the Related Art
A conventional rubber product with lips will be described with reference to FIGS. 6 to 10. FIG. 6 is a vertical cross-sectional view showing a rubber plug 101 having generally the same construction as that of a rubber product of the present invention with lips. A tubular body 102 includes a radially outwardly-projecting end 107, and a cylindrical extension portion 109 extends from this end along a longitudinal axis, and three horizontally-extending, disk-shaped lips 103, 104 and 105 are formed on an outer peripheral surface of the body 102, and are spaced at equal intervals from one another in the direction of the axis (that is, a=b). A flat open end of the outermost lip 105 defines the other end 108, and these portions are molded into an integral construction, using three molds which can be separated from one another in an upward-downward direction, with the main mold disposed between the upper and lower molds.
An inner peripheral surface 106 of the body 102 has a bellows-like cylindrical surface having a corrugated cross-section along the longitudinal axis X, and grooves 113, 114 and 115 of this corrugation correspond respectively to the lips, and the lips are formed on the outer peripheral surface of the body in such a manner that each lip and the corresponding groove have their respective median planes disposed in a common plane perpendicular to the axis. Similarly, ridges 116, 117 and 118 of the corrugated cross-section correspond respectively to annular constricted portions 111 and 112 each spacing the adjacent lips from each other in the axial direction.
The above rubber product with the lips is produced by an injection molding method using a three-stack-type mold as shown in FIG. 7. The upper mold A has a pin D for molding the inner peripheral surface 106 of the rubber plug 101, and this pin D extends vertically through a mold cavity in the centrally-disposed main mold B, and is mated at its lower end with an upper flat surface of the lower mold C.
For removing the injection molded product from the stack-type mold comprising the three mold portions which can be separated from one another, the upper mold A is first withdrawn, and then the lower mold C is removed, and finally the molded product is upwardly pushed out of the central main mold B. Particularly, in the final product removal step, a jet of air is applied to the lower end of the product, or a pushing force is applied to this lower end by a projecting member such as a pin, in an upward direction as indicated by a void arrow in FIG. 8. By this pushing force, the molded product is pushed upward step by step in such a manner that because of their elastic deformation, the three radially outwardly-projecting, disk-shaped lips 103, 104 and 105 are disengaged from their respective mold cavities, and are sequentially engaged in the upwardly-disposed mold cavities.
In this case, the three horizontally-extending lips of the product are spaced at equal intervals along the axis X of the body, and therefore a considerable frictional resistance is involved when the lips are sequentially engaged in the upwardly-disposed mold cavities in a step-by-step manner. That portion of each lip, which produces the maximum frictional resistance during this process, is indicated by a point Z in FIGS. 8 and 10 (which are cross-sectional views) for the simplicity of the description although this portion is actually annular.
In the first push-out step in which the uppermost lip 103 is disengaged from the main mold B, six maximum frictional resistance points (6.Z) develop on the product 100 as shown in FIG. 8. Then, when the second lip 104 is to be disengaged from the main mold B, four maximum frictional resistance points (4.Z) develop as shown in FIG. 9. Then, when disengaging the remaining lip 105 from the main mold so as to completely remove the product from the mold, two maximum frictional resistance points (2.Z) develop as shown in FIG. 10.
In the push-out step shown in FIGS. 8 and 9, the plurality of lips are simultaneously elastically deformed to thereby produce the large frictional resistance, and then are engaged respectively in the upwardly-disposed mold cavities, and therefore the considerable pushing force is required, so that the production efficiency is lowered, and besides there is a possibility that the molded product is subjected to damage such as the formation of a crack in the proximal end of the lip connected to the body, and the separation of the lip from the body. Particularly, in view of the fact that this operation is effected immediately after the product is molded, that is, when the product is not yet completely cured, this is a problem which can not be ignored from the viewpoint of the production.
Therefore, in order to avoid such a problem, it is desired that the mold should be so designed that the force required for one push-out operation can be reduced, that is, the mold removal resistance can be reduced.